Remove unneeded 1 pixel margin around side panels

Summary:
Remove unneeded 1 pixel margin around side panels, namely QAbstractScrollArea with property _kde_side_panel_view set to true.
In order to be able to still draw a vertical line on the side of the list, a one pixel margin is kept, on this side only, using SE_FrameContent subelementRect
The viewport background is kept to QPalette::Base, as for regular lists.
The logic in polishScrollArea has been adjusted accordingly

Differential Revision: https://phabricator.kde.org/D22138